Along with rapid advances in cellular phones, notebook computers and other portable electronic devices, there has been increasing demand for higher capacities in the batteries used as main power supplies and back-up power supplies for these, and attention has focused on lithium-ion secondary batteries and other nonaqueous electrolyte solution batteries, which have higher energy densities than nickel-cadmium batteries and nickel-hydrogen batteries.
Typical examples of electrolyte solutions for lithium-ion secondary batteries include nonaqueous electrolyte solutions comprising electrolytes such as LiPF6, LiBF4, LiN(CF3SO2)2 and LiCF3(CF2)3SO3 dissolved in mixed solvents consisting of ethylene carbonate, propylene carbonate and other high-permittivity solvents mixed with dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate and other low-viscosity solvents. Carbonaceous materials capable of storing and releasing lithium ions are primarily used as the negative electrode active materials of lithium-ion secondary batteries, and typical examples include natural graphite, artificial graphite, amorphous carbon and the like. Metal and alloy-based negative electrodes are also known that use silicon, tin and the in order to achieve higher capacities. Transition metal composite oxides capable of storing and releasing lithium ions are primarily used as the positive electrode active materials, and typical examples of transition metals include cobalt, nickel, manganese, iron and the like.
Because such lithium-ion secondary batteries use highly active positive and negative electrodes, charge-discharge capacities may be reduced by side reactions between the electrodes and the electrolyte solution. There are also concerns about safety when the battery is overcharged. To resolve these problems, various studies have been conducted on nonaqueous solvents and electrolytes.
Patent Document 1 discloses a technique for controlling battery swelling during high-temperature storage without reducing the battery capacity by including an aromatic ester compound in a nonaqueous electrolyte solution.
Patent Document 2 discloses a technique for controlling battery deterioration during high-temperature storage by using a nonaqueous electrolyte solution containing both an aromatic compound and an isocyanate compound, together with a positive electrode containing a specific amount of moisture in a lithium-ion secondary battery.
Patent Document 3 discloses a technique for suppressing high-resistance film formation by Li borate salts on the positive electrode, improving battery swelling during high-temperature storage and improving the cycle characteristics by including an aromatic compound together with various Li borate salts such as LiBF4 in a nonaqueous electrolyte solution.
[Patent Document 1] Japanese Patent Application Laid-open No. 2003-243026
[Patent Document 2] Japanese Patent Application Laid-open No. 2011-028860
[Patent Document 3] Japanese Patent Application Laid-open No. 2006-216378